Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
The temperature at which a pure organic substance changes from the liquid phase to the gas phase is known as the boiling point. A liquid's boiling point can be determined using the capillary method, where an inverted capillary is placed in the liquid of interest and the liquid is heated. As the temperature increases, the air in the capillary escapes and is replaced by the vapor of the liquid. The vapor pressure in the capillary increases with temperature. Once it exceeds the atmospheric pressure, the vapor escapes the capillary in a stream of bubbles. When the heat is removed, the liquid cools, and the vapor pressure in the capillary decreases. When the vapor pressure reaches the atmospheric pressure, the liquid begins to fill the capillary. The temperature at which this occurs is the boiling point.
| Boiling point temperature (°C) | ||
| Bubbles | Liquid in capillary | |
| Acetone | ||
| Ethanol | ||
In this experiment, we measured the boiling point of acetone to be 56°C, which compares well to the reported value. Similarly, the boiling point of ethanol was measured to be 78°C. Errors in the boiling point measurement can be attributed to many experimental errors, such as heating the water bath too rapidly, or poor alignment of the thermometer and sample.
The boiling point of an organic substance is directly related to its structure, where stronger intramolecular forces result in a higher boiling point as molecules are able to hold onto each other and remain in the liquid phase longer. The higher boiling point for ethanol is observed due to the OH structure that causes hydrogen bonding between the molecules. Acetone has a polar CO double bond, which results in dipole-dipole forces. Since hydrogen bonding is stronger than dipole-dipole forces, ethanol has a higher boiling point.
Additionally, ethanol has a lower molecular weight than acetone. However, molecular weight has less of an impact on the boiling point than the molecular structure. For example, butane is a gas at room temperature and pressure, as it has a boiling point lower than 25°C. Ethanol has a slightly lower molecular mass than butane, but it is liquid at room temperature and, therefore, has a boiling point higher than room temperature. This is due to the hydrogen bonding between the ethanol molecules, which is stronger than the van der Waals forces between the butane molecules.
The temperature at which a pure organic substance changes from the liquid phase to the gas phase is known as the boiling point. A liquid's boiling point can be determined using the capillary method, where an inverted capillary is placed in the liquid of interest and the liquid is heated. As the temperature increases, the air in the capillary escapes and is replaced by the vapor of the liquid.
The vapor pressure in the capillary increases with temperature. Once it exceeds the atmospheric pressure, the vapor escapes the capillary in a stream of bubbles. When the heat is removed, the liquid cools, and the vapor pressure in the capillary decreases.
When the vapor pressure reaches the atmospheric pressure, liquid begins to fill the capillary. The temperature at which this occurs is the boiling point. In this experiment, you will measure the boiling point of two organic solvents using the capillary method.
Before you start the lab, be sure to wear the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves. This experiment must be conducted in a hood. First, attach a small test tube to the thermometer using a rubber band and secure the thermometer in the clamp.
Then, obtain acetone and bring it back to your hood. Using your glass pipette and a bulb, measure one milliliter of acetone and transfer it into the small test tube. Align the thermometer end so that it is level with the acetone in the test tube, keeping them close to each other.
Now, invert a capillary tube and place it in the test tube so that the open end is facing downwards. Using your 250-milliliter beaker, create a water bath by adding approximately 180 milliliters of water. Place the water bath on the hotplate and lower the thermometer and test tube into the water bath.
Turn on the hotplate to the lowest setting, about 30 degrees. Then, slowly increase the temperature on the hotplate by 10 to 20 degrees Celsius every 10 minutes and closely observe the liquid in the test tube. When you start to see occasional bubbles in the liquid, increase the heat setting by only 5 degrees every 10 minutes, as the bubbles are an indication that the temperature is close to the boiling point.
Closely observe the capillary tube inside the test tube. Report the temperature at which a rapid and continuous stream of bubbles comes out of the capillary. This is the preliminary boiling point.
At this point, turn off the heat and allow the water bath to cool. Keep observing the capillary as the production of bubbles decreases until no bubbles emerge from the capillary. The liquid will then start to rise in the capillary.
Record the temperature at which this occurs, as this is the boiling point temperature of the liquid. Next, label a clean 25-milliliter beaker as organic waste'Once the water bath has cooled to about 35 degrees Celsius, remove the thermometer and test tube from the bath. Then, detach the test tube from the thermometer and pour the acetone into the waste beaker.
Now, use a clean test tube and capillary and repeat the experiment using one milliliter of ethanol. After you have measured the boiling point of ethanol, allow the water bath to cool. Once it has cooled sufficiently, remove the test tube and thermometer from the water bath, detach the test tube from the thermometer, and pour the ethanol into the organic waste beaker.
Dispose of the water from the water bath down the sink and place the capillaries and test tubes in the glass disposal. Dispose of the organic waste in the container provided by your instructor. Then, wash all of your glassware with detergent and water.
Always wash your hands before leaving the lab. In this experiment, we measured the boiling point of acetone to be 56 degrees Celsius, which compares well to the reported value. Similarly, the boiling point of ethanol was measured to be 78 degrees Celsius.
Errors in the boiling point measurement can be attributed to many experimental errors, such as heating the water bath too rapidly, or poor alignment of the thermometer and sample. The boiling point of an organic substance is directly related to its structure, where stronger intramolecular forces result in a higher boiling point as molecules are able to hold onto each other and remain in the liquid phase longer. The higher boiling point for ethanol is observed due to the OH structure that causes hydrogen bonding between the molecules.
Acetone has a polar CO double bond, which results in dipole-dipole forces. Since hydrogen bonding is stronger than dipole-dipole forces, ethanol has a higher boiling point. Additionally, ethanol has a lower molecular weight than acetone.
However, molecular weight has less of an impact on boiling point than the molecular structure. For example, butane is a gas at room temperature and pressure, as it has a boiling point lower than 25 degrees Celsius. Ethanol has a slightly lower molecular mass than butane but is liquid at room temperature and, therefore, has a boiling point higher than room temperature.
This is due to the hydrogen bonding between the ethanol molecules, which is stronger than the van der Waals forces between the butane molecules.
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